1,253 research outputs found
Adaptive approximate Bayesian computation for complex models
Approximate Bayesian computation (ABC) is a family of computational
techniques in Bayesian statistics. These techniques allow to fi t a model to
data without relying on the computation of the model likelihood. They instead
require to simulate a large number of times the model to be fi tted. A number
of re finements to the original rejection-based ABC scheme have been proposed,
including the sequential improvement of posterior distributions. This technique
allows to de- crease the number of model simulations required, but it still
presents several shortcomings which are particu- larly problematic for costly
to simulate complex models. We here provide a new algorithm to perform adaptive
approximate Bayesian computation, which is shown to perform better on both a
toy example and a complex social model.Comment: 14 pages, 5 figure
On the Geometry of Supersymmetric Quantum Mechanical Systems
We consider some simple examples of supersymmetric quantum mechanical systems
and explore their possible geometric interpretation with the help of geometric
aspects of real Clifford algebras. This leads to natural extensions of the
considered systems to higher dimensions and more complicated potentials.Comment: 18 page
On Chern-Simons Quivers and Toric Geometry
We discuss a class of 3-dimensional N=4 Chern-Simons (CS) quiver gauge models
obtained from M-theory compactifications on singular complex 4-dimensional
hyper-Kahler (HK) manifolds, which are realized explicitly as a cotangent
bundle over two-Fano toric varieties V^2. The corresponding CS gauge models are
encoded in quivers similar to toric diagrams of V^2. Using toric geometry, it
is shown that the constraints on CS levels can be related to toric equations
determining V^2.Comment: 14pg, 1 Figure, late
Coeducational ideas In physical education teachers: Pshycometric properties of a scale
Ante la falta de cuestionarios o escalas que evaluaran los diferentes aspectos coeducativos que caracterizan al profesorado de Educación Física y la necesidad de conocer cuál es la concepción que tienen sobre este modelo, se desprende el objetivo de este trabajo, como el de evaluar las propiedades psicométricas mediante un instrumento que hemos denominado Escala sobre el pensamiento coeducativo del profesorado de EF para valorar las opiniones respecto a la coeducación y la metodología que se utiliza en sus clases. La muestra estuvo compuesta por 213 profesores, 133 hombres y 43 mujeres. La escala fue aplicada mediante papel, envío de correo electrónico y plataforma moddle. El análisis de los datos muestra unos resultados adecuados en cuanto
a estructura factorial, consistencia interna y tipos de validez. Se concluye que esta Escala representa un instrumento válido y fiable para analizar las características coeducativas del profesoradoBecause of the absence of questionnaires or scales which will assess different coeducative aspects that are characteristics of PE teacher and need to know which is the conception that they have about this model, it is deduced the aim of this work as that which evaluate the psychometric properties through a tool called the Scale of coeducational ideas in physical education teachers to value the opinions about coeducation and the methodology used in their classes. The sample was composed by 213 teachers, 133 men and 43 women. The questionnaire was applied by means of paper, electronic mail shipment and platform moddle. The data analysis shows appropriate results in terms of factor structure, internal consistency and validity types. We conclude that the escale represents a valid and reliable instrument to analyze the coeducative characteristics of the teacher
Muon spin rotation and neutron scattering study of the non-centrosymmetric tetragonal compound CeAuAl3
We have investigated the non-centrosymmetric tetragonal heavy-fermion
compound CeAuAl3 using muon spin rotation (muSR), neutron diffraction (ND) and
inelastic neutron scattering (INS) measurements. We have also revisited the
magnetic, transport and thermal properties. The magnetic susceptibility reveals
an antiferromagnetic transition at 1.1 K with a possibility of another magnetic
transition near 0.18 K. The heat capacity shows a sharp lambda-type anomaly at
1.1 K in zero-filed, which broadens and moves to higher temperature in applied
magnetic field. Our zero-field muSR and ND measurements confirm the existence
of a long-range magnetic ground state below 1.2 K. Further the ND study reveals
an incommensurate magnetic ordering with a magnetic propagation vector k = (0,
0, 0.52) and a spiral structure of Ce moments coupled ferromagnetically within
the ab-plane. Our INS study reveals the presence of two well-defined crystal
electric field (CEF) excitations at 5.1 meV and 24.6 meV in the paramagnetic
phase of CeAuAl3 which can be explained on the basis of the CEF theory.
Furthermore, low energy quasi-elastic excitations show a Gaussian line shape
below 30 K compared to a Lorentzian line shape above 30 K, indicating a
slowdown of spin fluctuation below 30 K. We have estimated a Kondo temperature
of TK=3.5 K from the quasi-elastic linewidth, which is in good agreement with
that estimated from the heat capacity. This study also indicates the absence of
any CEF-phonon coupling unlike that observed in isostructural CeCuAl3. The CEF
parameters, energy level scheme and their wave functions obtained from the
analysis of INS data explain satisfactorily the single crystal susceptibility
in the presence of two-ion anisotropic exchange interaction in CeAuAl3.Comment: 28 pages and 17 figure
Interacting Multiple Try Algorithms with Different Proposal Distributions
We propose a new class of interacting Markov chain Monte Carlo (MCMC)
algorithms designed for increasing the efficiency of a modified multiple-try
Metropolis (MTM) algorithm. The extension with respect to the existing MCMC
literature is twofold. The sampler proposed extends the basic MTM algorithm by
allowing different proposal distributions in the multiple-try generation step.
We exploit the structure of the MTM algorithm with different proposal
distributions to naturally introduce an interacting MTM mechanism (IMTM) that
expands the class of population Monte Carlo methods. We show the validity of
the algorithm and discuss the choice of the selection weights and of the
different proposals. We provide numerical studies which show that the new
algorithm can perform better than the basic MTM algorithm and that the
interaction mechanism allows the IMTM to efficiently explore the state space
On Non Commutative G2 structure
Using an algebraic orbifold method, we present non-commutative aspects of
structure of seven dimensional real manifolds. We first develop and solve
the non commutativity parameter constraint equations defining manifold
algebras. We show that there are eight possible solutions for this extended
structure, one of which corresponds to the commutative case. Then we obtain a
matrix representation solving such algebras using combinatorial arguments. An
application to matrix model of M-theory is discussed.Comment: 16 pages, Latex. Typos corrected, minor changes. Version to appear in
J. Phys.A: Math.Gen.(2005
Ergodicity, Decisions, and Partial Information
In the simplest sequential decision problem for an ergodic stochastic process
X, at each time n a decision u_n is made as a function of past observations
X_0,...,X_{n-1}, and a loss l(u_n,X_n) is incurred. In this setting, it is
known that one may choose (under a mild integrability assumption) a decision
strategy whose pathwise time-average loss is asymptotically smaller than that
of any other strategy. The corresponding problem in the case of partial
information proves to be much more delicate, however: if the process X is not
observable, but decisions must be based on the observation of a different
process Y, the existence of pathwise optimal strategies is not guaranteed.
The aim of this paper is to exhibit connections between pathwise optimal
strategies and notions from ergodic theory. The sequential decision problem is
developed in the general setting of an ergodic dynamical system (\Omega,B,P,T)
with partial information Y\subseteq B. The existence of pathwise optimal
strategies grounded in two basic properties: the conditional ergodic theory of
the dynamical system, and the complexity of the loss function. When the loss
function is not too complex, a general sufficient condition for the existence
of pathwise optimal strategies is that the dynamical system is a conditional
K-automorphism relative to the past observations \bigvee_n T^n Y. If the
conditional ergodicity assumption is strengthened, the complexity assumption
can be weakened. Several examples demonstrate the interplay between complexity
and ergodicity, which does not arise in the case of full information. Our
results also yield a decision-theoretic characterization of weak mixing in
ergodic theory, and establish pathwise optimality of ergodic nonlinear filters.Comment: 45 page
A population Monte Carlo scheme with transformed weights and its application to stochastic kinetic models
This paper addresses the problem of Monte Carlo approximation of posterior
probability distributions. In particular, we have considered a recently
proposed technique known as population Monte Carlo (PMC), which is based on an
iterative importance sampling approach. An important drawback of this
methodology is the degeneracy of the importance weights when the dimension of
either the observations or the variables of interest is high. To alleviate this
difficulty, we propose a novel method that performs a nonlinear transformation
on the importance weights. This operation reduces the weight variation, hence
it avoids their degeneracy and increases the efficiency of the importance
sampling scheme, specially when drawing from a proposal functions which are
poorly adapted to the true posterior.
For the sake of illustration, we have applied the proposed algorithm to the
estimation of the parameters of a Gaussian mixture model. This is a very simple
problem that enables us to clearly show and discuss the main features of the
proposed technique. As a practical application, we have also considered the
popular (and challenging) problem of estimating the rate parameters of
stochastic kinetic models (SKM). SKMs are highly multivariate systems that
model molecular interactions in biological and chemical problems. We introduce
a particularization of the proposed algorithm to SKMs and present numerical
results.Comment: 35 pages, 8 figure
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